Microlens Arrays Produced by Soft Lithography
Technique enables potential alternative to photolithography-based arrays.
Daniel S. Burgess
A team of scientists at Sungkyunkwan University in Suwon and at LG Chem Ltd.’s Research Park in Taejon, both in South Korea, has fabricated two-dimensional arrays of microlenses by soft lithography, using spin-cast polystyrene beads as a template. The technique may have applications in the production of microlens arrays for use with organic LEDs (OLEDs) and other photonics systems.
Electron microscope images display a microlens array created in photopolymer by soft lithography, using polystyrene beads as a template. The approach may have applications in the production of arrays for use with organic LEDs, lasers, fiber optics and other photonics systems. Reprinted with permission of Langmuir.
Duk-Young Jung, an associate professor of chemistry at the university, said that the integration of ordered micro-optical elements into the structure of OLEDs has been shown to increase their output by guiding radiation out of the devices that otherwise would be trapped by waveguiding and total internal reflection, and lost. He noted that a number of approaches exist for the production of microlens arrays, but that most involve a high-quality template produced by photolithography.
“Replica molding using soft lithography is an inexpensive, reproducible and universal technique applicable to a large area of surface in a short time,” he said.
In their demonstration of the technique, the researchers first employed spin casting to deposit 325- or 454-nm-diameter polystyrene beads on plasma-treated glass substrates, optimizing the spinning speed, spinning time and concentration of beads in the aqueous solution to yield a closely packed monolayer. The beads, which were found to be highly ordered over a 4 × 4-cm area, were heated for one hour at 70 °C so that they would melt slightly and would adhere to the substrate. Negative molds 4.5 mm in thickness were then produced by pouring polydimethylsil-oxane over the hexagonal arrays, exposing the material to a vacuum for 30 minutes to eliminate gas bubbles and curing it for 40 minutes at 70 °C.
To fabricate the microlens arrays, they loaded the cured and cleaned molds with a urethane-based UV-curable photopolymer. A high-pressure mercury lamp served as the irradiation source. An analysis using atomic force microscopy revealed that the arrays largely were free of defects, but that incomplete penetration of the viscous photopolymer solution into the voids of the mold had resulted in a loss of height compared with the template — in the case of the 454-nm beads, the original features were 103 nm high and the replicas, 95 nm.
Jung explained that this problem with loss of feature height, which the investigators attributed to the hydrophobic characteristics of the template, becomes more severe as the size of the polystyrene beads decreases. He added that the technique has trouble fabricating microlenses that are not closely spaced in the array.
The scientists are exploring the optical properties of the microlenses, he said, and hope to obtain a laser diffraction pattern of the arrays.
Langmuir, Aug. 15, 2006, pp. 7358-7363.
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